Boss/Ibanez flip-flop switching

[poiureza]

Yes, that topic again

I read the geofex articles and the similar premier guitar one and I understand most of it.
https://www.premierguitar.com/jfet-switching
The part I don't get is the "blip signal".

How does that work ?
When you press the switch you shunt the PS to ground, yet the circuit is connected there via 220pF caps in series so nothing should happen.
Are we like pulling those 220pF empty when switching and this affects the other side of the caps ? 
We have 9V through 1M+100R resistor on one leg to GND and 220pF through 100R on the other leg to GND, so the latter one has least resistance and wins or something ?
In that case why all the 9V hookup ?  Couldn't we just switch the 220pF directly to ground with a momentary DPST switch ?
Eventually I also don't understand why it needs that 100R and 10nF cap.

I do understand that it's a matter of transients (voltage ? current ?) as in steady state nothing should happen obviously.

[antonis]

When you press the switch you shunt the PS to ground, yet the circuit is connected there via 220pF caps in series so nothing should happen.

Actually, it's connected in shunt..
With footswitch closed, 2 X 220pF common legs are effectivelly set at 90mV after a short-time delay of 5μs (5 x 10nF/100R time constant)

[Rob Strand]

You need to understand two processes:
- how the trigger pulse is formed
- how it toggles the state of the flip-flip

To understand trigger RC circuits you first need to identify the voltages at each end of the capacitor in the steady-state ie. the circuit is sitting there for a long time.

The 10nF cap is used to debounce the switch.  It's not really part of the trigger circuit.
Don't worry about the 100R.   It complicates the description without adding to the
main actions.  Basically limits the switch current.

Triggering:
- At the switch end with the switch off the 10n cap will charge to +9V.   
  That also means the point where the two 220pF connect is sitting at +9V.
- For the transistor that is on, the base will sit at 0.6V and for the transistor that is off the base
  will sit at 0V.    Those to voltages define the voltage on the other side of the 220pF cap.
- In rough terms the voltage on base side of the 220pF caps is 9V below the switch voltage.
- When you press the switch it discharges the 10n cap and the switch voltage will go to 0V.
- Now for the important point:  If the switch voltage is 0V and the base side of the 220pF caps
  is 9V below the switch voltage, that means the bases are pulled down to -9V.
- What that does is turns off *both* transistors.  Whatever transistor was on, it's now off.
   Upto this point is the trigger process.

Changing State:
- The tricky part now is why only the other transistor turns on.  This is the change of state process.
- The 470pF cap on the transistor that was on will have 0V at both ends.
-  When the transistor that was on gets turned off the collector voltage rises to 6V
   that forces the base of the transistor which was off to a positive voltage and it turns on.
   (It's 6V on the collectors not 9V because the 100k base resistors load down the 56k collector resistor.)
-  For the transistor that was off.   It was already off so the it's collector voltage doesn't change (yet).
   That also means the voltage on the 470pF cap at the collector of this transistor doesn't change.
   So the this 470pF cap doesn't have an effect on the other transistor (the one that was on).
- In other words only the transistor that was off gets turned on.

You might need to run over that a few times.  These circuits can do your head in when you first see them.
Draw voltages on the circuit at each step. Remember caps don't change voltage instantly. 

If you want a simple trigger circuit to understand first, read-up on the NE555 monostable.  Here's the trigger circuit,

[antonis]

After Rob's literal analysis, you're ready to implement a custom made flasher for your bike..

[poiureza]

Thank you for the valuable information Rob.
I don't fully understand the changing state (I keep that for later)

This is the part that I have to chew on :

- Now for the important point:  If the switch voltage is 0V and the base side of the 220pF caps
  is 9V below the switch voltage, that means the bases are pulled down to -9V.

So changing voltage on one side affects the other end of the cap.

Now I think about it, that's exactly how it works for an AC signal through a cap.
Changing DC level is sort of an AC signal
Dooh ...

[antonis]

https://engineer-educators.com/topic/the-rc-time-constant/

[Rob Strand]

Now I think about it, that's exactly how it works for an AC signal through a cap.
Changing DC level is sort of an AC signal
Dooh ...

Yes, that's a valid way to think of it.

In textbooks they use 'transient analysis' to describe these circuits.  A classical example is the link antonis posted.   From that link, if you look the voltage across the resistor instead of the cap you get the spiked waveform shown on the NE555 trigger circuit.  All the same thing. 

The final voltages on the caps are like the DC voltages you get when the caps are removed (look at antonis's link then the NE555 circuit).   For a circuit with a switch you have the DC voltages when the switch is open and DC voltages when the switch is close.  When the switch is open for a while then closed the caps charge up and the voltages move (transition) from the one set of DC voltages to another.    With the flip-flip circuit it's not that simple because the pulse circuit has one set of transient voltages then that feeds into the flip-flip which has another set of transient voltages.  It's like two separate circuits with two separate actions - the pulse at the joint of the 220pF & 470pF caps starts the second action.

[poiureza]

Thanks again for the explanation.

I have since found many web articles about "inrush current", something I was totally unaware of (Series caps acting as a short while filling etc ...).  Seems connected to the phenomenon and the whole topic is certainly an eye opener to me.

PS : I have taken the bad habit of calling "AC" everything that is not DC, while we are indeed talking about transients here.

[GibsonGM]

My buddy (an engineer) had a HECK of a time getting me to stop calling anything non steady-state "AC".  To be true "AC", he explained, it must cross zero and alternate polarity (duh).   

We then have transients (voltage states not repeating on a regular basis), or 'pulsating DC' (ripple...) and so on we can use to classify other non steady voltage levels.   I found later that it really helped to understand what was happening in a circuit, like a 555 timer, to stop the "AC" talk and think more precisely ("2/3 Vcc"), so it was a positive thing!     

But the VERY hardest challenge he had was to get me to think in terms of current instead of voltage, LOL!!

[antonis]

To be true "AC", he explained, it must cross zero and alternate polarity (duh).

But "zero" is wherever WE ARBITRARY designate x-axis, isn't it Sir..??

[DIY Bass]

To be true "AC", he explained, it must cross zero and alternate polarity (duh).

But "zero" is wherever WE ARBITRARY designate x-axis, isn't it Sir..??

This sounds to me like the kind of discussion that ends up talking about masses moving along frictionless surfaces and perfect spheres moving through airless spaces under the force of gravity.

[iainpunk]

After Rob's literal analysis, you're ready to implement a custom made flasher for your bike..

idk how the laws are in your area, but here in the Netherlands, youre not allowed flashers on any road vehicle

To be true "AC", he explained, it must cross zero and alternate polarity (duh).

But "zero" is wherever WE ARBITRARY designate x-axis, isn't it Sir..??

This sounds to me like the kind of discussion that ends up talking about masses moving along frictionless surfaces and perfect spheres moving through airless spaces under the force of gravity.

no, its not that theoretical. AC is still AC, even if there is a greater DC offset. just look at the fourier transform. 0-crossing isn't required for it to be AC. its just a compound signal, AC and DC added together, just like phantom power for some microphones.

a better statement would be that everything is AC, and a DC offset is just a really low frequency standing nearly still at + or -90 degrees. nothing is forever, DC doesn't exist at all. its all lies, birds don't exist, they are government drones. gravity doesn't exist either, its universal acceleration of the plane were on, happening in the 5th or 9th directional dimension (which we can't observe) Australia is also a lie, their all CIA actors... do you really believe people lost a war against birds? that's exactly what lizard people want you to think! lizards and birds have been enemies for ages now, and the benevolent bird people are actually on our side!

cheers

[amptramp]

After Rob's literal analysis, you're ready to implement a custom made flasher for your bike..

idk how the laws are in your area, but here in the Netherlands, youre not allowed flashers on any road vehicle

To be true "AC", he explained, it must cross zero and alternate polarity (duh).

But "zero" is wherever WE ARBITRARY designate x-axis, isn't it Sir..??

This sounds to me like the kind of discussion that ends up talking about masses moving along frictionless surfaces and perfect spheres moving through airless spaces under the force of gravity.

no, its not that theoretical. AC is still AC, even if there is a greater DC offset. just look at the fourier transform. 0-crossing isn't required for it to be AC. its just a compound signal, AC and DC added together, just like phantom power for some microphones.

a better statement would be that everything is AC, and a DC offset is just a really low frequency standing nearly still at + or -90 degrees. nothing is forever, DC doesn't exist at all. its all lies, birds don't exist, they are government drones. gravity doesn't exist either, its universal acceleration of the plane were on, happening in the 5th or 9th directional dimension (which we can't observe) Australia is also a lie, their all CIA actors... do you really believe people lost a war against birds? that's exactly what lizard people want you to think! lizards and birds have been enemies for ages now, and the benevolent bird people are actually on our side!

cheers

This certainly clarifies Boss/Ibanez switching.